ENH: optimize area of top surface

Signed-off-by: salt.wei <salt.wei@bambulab.com>
Change-Id: Ia3d1be6ad3b9f55ee1acda14d0be955055f6dfcb
(cherry picked from commit 448fd3043950613a4d91df256fa9ee52a57fcaca)

src/libslic3r/Layer.hpp

@@ -184,6 +184,8 @@ public:
 
     //BBS
     void simplify_extrusion_path() { for (auto layerm : m_regions) layerm->simplify_extrusion_entity();}
+    //BBS: this function calculate the maximum void grid area of sparse infill of this layer. Just estimated value
+    coordf_t get_sparse_infill_max_void_area();
 
 protected:
     friend class PrintObject;